Ethanol is an attractive alternate transportation fuel to replace at least a part of petroleum (Kheshgi, et al., 2000, Wooley, et al., 1999). Although ethanol is currently produced in the U.S. by fermenting glucose from cornstarch using Saccharomyces cerevisiae (Bothast, et al., 2005), expanding this process to produce a large fraction of the automotive fuel requirement would adversely impact the food and feed industry.
Lignocellulosic biomass is an attractive alternative feedstock that can be fermented to ethanol after appropriate pretreatment without impacting food and feed supply (Wyman, et al., 2003, Zaldivar, et al., 2001). In contrast to cornstarch, biomass contains significant amounts of pentose sugars that are recalcitrant to fermentation by yeast.
Conversion of complex sugars to ethanol requires microbial biocatalysts that effectively ferment both hexose and pentose sugars. Towards this goal, recombinant organisms have been developed in which heterologous genes were added to platform organisms such as yeasts, Z. mobilis and E. coli. For example, recombinant ethanologenic Escherichia coli containing the pdc and adh genes from Zymomonas mobilis ferment both hexoses and pentoses to ethanol at high rate and yield (Ingram, et al., 1999). In addition, genetic engineering of yeast and Z. mobilis by adding genes for pentose utilization has yet to yield a biocatalyst that matches the pentose fermentation characteristics of the recombinant ethanologenic Escherichia coli (Kuyper, et al., 2005, Mohagheghi, et al., 2004).